Free-Space–Coupled Frequency-Locked Microtoroid Sensors for Ultra-Sensitive Label-Free Chemical and Biological Detection
Whispering gallery mode (WGM) microresonators have emerged as powerful platforms for label-free chemical and biological sensing due to their ultra-high quality factors and strong evanescent field interactions. Frequency-locked optical whispering evanescent resonator (FLOWER) systems have demonstrated single-molecule sensitivity and zeptomolar detection limits in liquid environments. Conventional implementations, however, rely on tapered fiber coupling, which can limit robustness, scalability, and practical deployment. Here, we describe free-space–coupled frequency-locked microtoroid sensor architectures designed to eliminate fragile tapered fibers while maintaining high sensitivity. Free-space optical excitation of ultra-high-Q silica microtoroids provides a promising route toward compact and mechanically robust sensing platforms. We discuss the operating principles, system design considerations, and applications of these sensors for chemical and biological detection. These approaches may enable deployable WGM sensing systems for environmental monitoring, defense, and biomedical diagnostics.